Note: Descriptions are shown in the official language in which they were submitted.
~4~3279
The pre~en~ invention relates to a procesi~ or
treating brass-skimmings and similar waste materials fox
recovery of metallics and useful by-product~ therefrom
with minimal losses.
Whenever molten brass comes in contact with air
tor other ox~gen containing gases), the zinc aonten~ thereof
rapidly forms a ~urface layer o æinc oxide as an impurity.
Thus în the manufacture o virgin brass and in the sub~
se~uent foundry melting of the billets, a significant amount
o the zinc content o the brass i9 lost as zinc oxlde which
forms as a floating dross and additional signi~icant amounts
of the brass alloy itself iis lost in skimmin~ off the floating
impurities and fluxe~ prior to p~uring the molten brass. As
a result, it has been estimated that S~ of all melted brass
is lost to 3uch skimmings. The typical metallic values in
such skimmlng are in excess of 85~ (primarily copper and zinc
in alloyed form an~ zinc oxide).
It can ~e appreciated that s~ch losses represent
a significant economic aon~ideration, which the industry
since its inception has long sought to minimize by various
methods for recovering the metallic content ~f such skimmings.
Two major bras~ mills alone each generate an average Qf 400
to 750 ton~ of brass skimming~ a month respectively.
Over the years several d~fferent methods have been ~`
attempted to recover these losses. While some ~f these have
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succeeded to some degree in reducing the lo~ses, they all
still involve substantial loss of the metal content of the
brass ski~nings and are additionally quite costly. There
have been three principal prior art method3. Some secondary
manufacturers treat the brass ski~nings pyrometalurgically,
for example in a rotary furnace, tieing up the zinc oxide in
fluxes, thus losing 30% to 40% of the zinc content and 10 to
20~ of the copper content and additionally charging about
1% - 3~ of the market value of the brass nugge~-s re~uxnad or
the processing.
Recovery has also been achieved by mechanical means
by grinding the brass skimmings, suGh as by ball milling,
and separating the metals rom the impurities by hydraulic
clas~ification. This re~overs up to 90% of the material in
the brass skimmings present in metallic form with the washed
out fines being discarded as waste constituting essentially
all the zinc oxlde mlxed with other debris (including carbon
and silica) and as much as 10% to 14% by weight of copper.
In a third met~od, the brass skimmings are smelted
and the zinc oxide layer is blown off as waste with a 1% to
3% copper loss. Mo~t of the zinc oontent of the alloy is
converted to zina oxida during this process and is also l~st
during the blowing process. The brass is then cast into an
an~de with khe copper content ~hereof purified and recovered
separately by ~lectoly~ic deposition; see for example U.S.
Patent 1,920,820.
It i~ an object of the present in~ention to recover
substantially all of the metal values of brass ski~nings ~or
of similar alloys having copper and zinc as significant com-
p~nents), thus realiziny typically a 10 to 20 fold reduction
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in lo~ses over prior processe~ and at the same time achieving
thesa results at greatly reduced proces~ing co~ts.
It is a further object of the present invention to
convert the zinc oxide, previously regarded and treated a~
a wa te product, so as to recover the æinc value in ~he form :~
of a usefu~ by-product.
In the specification and in the accompanying draw
ings these and other objects and advantage~ will become more
apparent, and additionally there are described and shown
illustrative embodiments of the invention and various modifi-
cations thereof are suggested, but it i5 to be understood
that these are not intended to be exhaustive, but on the
contrary are given for purposes of illustration in order
that others skilled in the art may more fully understand the
invention so that they may modlfy and adapt it in various
f~rms, each as may be best suited to the conditions to a
particular use,
In the drawings: ~-
Figure 1 is a schematic representation tpartially .
in the form of a flQw diagram) of apparatus for carrying ~ut
a preferred process emb~dying the present invention;
Figure 2 is a schematic representation showing a
partial modification of the apparatus depicked in ~igure l;
and
Figure 3 is a flow diagram representing an alterna~
tive pr~cess emb~dyin~ the present invention. :
The objec~ and advantages o~ this invention are
achieved by a strikingly simple and yet heretofore unobvious
application of chemical methods by which 98 t~ 99.S~ of the :~
total metallic values can readily be reco~ered ~this alon~
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givlng huge economic value to the process) and urth~r at a
proces~ing cost typically ~ignificantly less t~an prior art
processes.
In its simplest form, the present invention involves
crushing the brass skimmings, e.g. by hammer milling, to
loo~en and expose zinc oxide (a white powder) to subsequent
attack by concentrated hydrochloric acid or slightly diluted
sulfuric acid under conditions which effect conversion o
the zinc oxide to a solution of æinc chloride or zinc sulfate
but which are insufficient to permlt any e~fective attack of
the copper alloy particles ~such conditions typically involv-
ing a reaction limited to less than one minute's duration
and at a pH less than 3.5 and preferably ranging fxom 0~1
to 1.5). .
Prior art methods o~ treating brass skimmings con-
sidered the zinc oxide as a waste product. The present
invention converts the zinc of the æinc ~xide to a useful
by-product and does so with an efficiency such that all ~f
the metallic values arè recovered with practically no lossesc
2~ Zinc chloride is useful in ~alvanizing iron, in soldering
fluxes, in medical preparations, as a catalyst, and wQod
preservative, among many other uses. Zinc chloride in the
past has often been produced from the 2inc skimmings fr~m
galvanizing baths which typically have ammonia present in
the fluxes. The present invention gives ammonia-free zinc
chloride, which is highly desirable. Zinc sulfate finds
uses as a preservative, in b~eaching paper, as a fungicide,
and a dietary supplement.
In the broader aspects of this invent~on, the term
"brass skimmlngs" is intended ~o include the s~immings rom
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the melt from an alloy in w~lich copper and zinc ~orm signifi-
cant constituents such that the dross included in said bras~
skimmings has in addition to said alloy zinc oxide as a
~ignificant component. The compo3ition of the brass skimmings
will depend greatly upon not only the composition of the alloy
melt, but also upon the mill practices, including the fluxes
used and the skill of ~he skimmer. The higher the zinc con-
tent of the alloy and the longer the melt is exposed to the
atmosphere, the greater will be the amount of scum ~loating
on the alloy, particularly when the zinc content of the alloy
i8 in excess o 20% and when significant amounts of fluxes
are added. Charcoal, coke, or some other carb~naceous
material, is the u~ual cover to pravent the zinc from vapor- :
izing from the melt and also to reduce the ~ormation of zinc
oxide. Other ~luxes used in an attempt to reduce the metallic
content of the scum can include common salt, borax, soda ash,
and even glass.
The cooled mass of brass skimmings is typically
run through with metallic shot, nuggets, s~eletons, and scab~.
2~ As a preliminary step in the present inventionl this mass
must be crushed at least to the extent required to loosen and
expo~e most of the zinc oxide. This can be su~ficiently
accompli~hed by a relatively quicX treatment by a hammer mill.
The lo~sened mass can then be passed through a number 4 mesh
screen which will typically retain 20% to 50%. This retained
material is 85% - 95% metal with the remainder consisting of
dirt and debris from the melting pr~cess which can incIude
part of the ~urnace lining, siIica, carbon, and other dirtO
This retained material is of sufficient purity and comp~sition
tha~ it can be directly recycled to the meltin~ furnaces of
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the mill or oundry Erom which it was received. It al~o ha
the advantage o having ~he alloy composition of the mill from
which it wa~ raceived~ If desired, thls retained material
can be cleaned by a quick exposure to acid, however this is
mainly for "sales appeal" ~y brightening the metal and result~
ing ln only a sligh~ up~rading of the purity.
The material passing through the screen can then
be charged directly ~o a reaction vessel a~ a controlled
rate. In a pilot operation, a 50~ gallon reaction vessel was
continuously charged at the rat~ of 5 tons per hour. A
commercial operation will typically charge at the rate of 20
tons per hour.
; Referring to Figure 1, the reaction vessel 10 is
charged by the feed device 12 receiving the minus ~4 mesh
material from the screen 14 which has already been crushed by
; the hammer mill 16. Acid is added to the reaction vessel la
through inlet 18 at a similarly controlled rate. If re~uired,
water is added to the reaction vesseI 10 through pipe 20D
The reaction mass in the vessel 10 is continu~usly mixed and
agitated b~ a~itator 22 driven by motor 24.
Depen~ing upon the by-product desired, either con-
cent~ated hy~rQchl~ric acid ~r concentrated sulfuric acid
ls added through inlet 18. The exothermic reaction of the
aci~ with the powdered zinc oxide present in the ~rass skim-
mings is very rapid and there~ore requires a reaation timeof less than a minute with no heat nor pressure being required~
The reaction time of the acid in attacking the metals present
in the brass skimmings is comparatively slow, particularly
since the relati~e surface area is so small due to th~
presence ~ the majori~y of the metal in the ~orm o~ nuggets,
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qhot, and other relatively large particles (even after the
aforesaid ~creening).
In this foregoing example of the preferred embodi-
ment, the bra~s skimmings are added to the reaction vessel
10 at the rate of 5 tons per houx. ~he p~ of the reaction
vessel i~ monitored by sensing device 26 whlch in turn
controls valve 28 thus metering in the acld through inlet
18 so as to keep the reaa~ion mass wi~hin a ~pecific pH
range.
~pplicant has determined that if the reaction mass
is kept withln the preferred pH range, then the reaction
ma~s is acid enough to rapidly attaok ef~ectively all of
the zinc oxide and yet is not so acid as unnecess~rily to
attack the metallics present in the brass skimmings. F.or ~ :
hydrochloric acid, the desirable pH ranye is from 0.1 to
little m~re than 1.0 with the optimum range being from 0.3
to 0.5. F~r sulphuric acid, the desired range is from 0.1
to l.S with the optimum range ~eing from 0.1 t~ 0.8. ~:
Although this pro~ess can be practiced out~ide these ranges,
20 it i8 generally undesirable. With too low a pH, there will
: be an ex~ess of acid, requ1rins the addition of zinc duRt
~or its-equivalent) ln order to neutralize the excess acid
overflowing the weir 30 from the reaction vessel 10 and to
c~nvert the copper salts (resulting from acid attack of the .
~rass metal) to the de3ired zinc salts and to precipitate
out metallic copperO On the other hand, with too hi~h a pH
there will be incompletely reacted zinc oxide contaminating
the reaction products exiting from the w~ir 30. As the pH
of 3.5 is approached, the reaction becomes too slow for
practical purposes, even if the residence time in the
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vesgel 10 i9 greatly increa~ed.
It is possible in the broader aspect~ of this inven-
tion to control the reaction by monitoring other parameters
or even by metered addition of pre-analyzed reactants; none
of which howev~r are as simple and convenient a~ the pH control
method.
The reaction product in thi~ preferred example
exits from the reac~ion vessel lO by a weir overflow 300
The unreacted heavier particles, mainly consisting of solid
metallics, settle to the bottom of the vessel lO and exit
through valve mechani~m 32. This can be a simple diaphragm
valve, a xotary valve, or even a double valve ~or batch
removal of solids.
The reaction wit~in vessel lO can be subject ~o
foaming. However this is normally not to~ serious a problem
which can be genexally controlled, if necessary, by proper
agitation or other known method~ including additive foam
suppressants. :'
The acid and crushe-d brass s~immings are added in
a conven~i~nal manner suficiently remote ~rom the weir o~r-
fl~w 30 to insure th~rough mixing and sufficient residence
tim~ ~ the reactants in the r~action zone of the vessel lO
to assure that the zin~ oxide .is converted to the æinc salt
before it can reach the weir overflow 30. The addition ~f
water from supply pipe 20 to the reaction vessel lO control-
led by valve 34 is intended mainly to keep the reaction mass
sufficiently dilute to insure that the ~inc salt reaction
product remains in solution. This is mainly required for zlnc
sulfate. ~;
The liquid ove~flGw from the weir 30 is mainly a
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zinc ~alt ~olution; however, it may con~ain minor amounts
of very ~ine metallic~, other contamlnants,, and perhaps
some other ma~al salts, normally the copper salt. Con~e-
qyently, (if necessary), this overflow can be subsequently
S treated by the addition of zinc dust to convert the other
more electropositive metals ~o their metallic ~orm thus
creating addi~ional zinc salts. Thi~ ~tep has been illus-
trated in the form of a precipitatox 36. Thereafter the
liquid mass is filtered or continuously centriuged, f~r
example in a separa~or 38, to remove the solid fines. These
solid contaminan~ will usually contain a large mass of dirt
with less than 1/2~ copper (by weight o~ the original chargs).
This copper contact is usually too small to be of concern,
or even of value for reclamation.
The sepaxated solids exiting through valve mechanism
32 of reaction vessel 10 are preferably subjected to a gentle
washing. In Figure 1 is illustrated an inclined screw con-
veyor 40 rlsing from an overflow wash tank 42 against a
countercurrent of wash water from spray device 440 I~ desir-
ed, the soluble salts washed from the solid metallics can
be reclaimed by combining the overflow fr~m ~he tank 42 via
the ~onduit 46 (indicated by dots as being an optional
alternative) to feed into conduit 48. In this case the wash
water 44 ~s minimized to avoid unnecessary dilution of the ' ,~
æinc salt ~olution. Conduit 48 leads from overflow weir 30
to the precipitator 36. ~ ,
Depending to a large extent upon the ~uality of
the brass ~kimmings received, the matexial recovered from
the screw conveyor 40 will c~n~ain 80% to 95% metal, up to
8% moisture, and the remainder will ~e dirt. Further purifi~
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cation has been found to be unnecessary. The recovered
material can ~e recycled dlrectly to the melting urnaceM.
Typically ~5~ to 99% or more of ~he zinc oxide i~
solubilized and separated from the metallics by conversion
to a useful zinc salt by-product. The contaminant~ in the
recovered metalR are signi~lcantly reduced with any remaining
amounts bein~ well within acceptable limits for recharging
to tha bras~ furnaces. The zinc salt ~olution by-product
i ammonia-free and has less than 10 parts per million of
copper. Since the zinc sa}ts result almost exclu~ively from
the æinc oxide and not from the zinc in the metallic alloy,
the alloy composition in the brass skimmin~s rema$ns ~ub-
stantially unchanged and can usually be recycled directly,
with little or no necessity for additional spel~ering.
It can be appreciated the present invention has a
significant anti-pollution advantage by converting a former
waste produc~ (i.e. contaminated zinc oxide) into a useful
by-pr~duct, A measure ~f the effectiveness and efficiency
of this invention is that the discarded cla~si~ication fines
and/or flue dust fr~m the various prior art proceases for
treating brass skimmings can eaonomically be processed by
the present ~nvention for recovery o the metallic values
an* conversion o~ the 2inc oxlde to the zinc salts.
The #4 screen after the hammer mill 16 can be
omitted.
In an alternative to the proces~ illustra~ed in
Figure 1, the hamm~r mill-16 and scree~ 14 can be replaced
by a ball mill S0 which crushes the brass skimmings ~nto
smaller particles for even more efficient and effective
treatmentO After the solidified brass skimmings are run
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relatively quic~ly ~hrough the ball mill and acid-treated,
th0 separated ~ettled solids can be passed over a 50 to 70
me~h screen and the ~olids retained thereon will be es~en-
tlally pure metal. The minus 50 to 70 me~h materials are
S about 80~ metallic which can be dried and char~ed directly to
a rotary furnace and even directly to an electric furnace
(depending upon the u~er's know how). Altern~tively, it can
be bri~uetted, advan~ageously with br~s turnings, or aiding
in charging the electric ~urnaces. ~he carbon and fluxes
pre~ent in the dirt is advantageous for recyclin~ and the
silica and other dirt i9 probably eventually removed as fines
fr~m the separator 38 a~ter several recyclings.
In a comparison ~f the present invention with the
prior art practiced by the smelting and refining companies,
the invention process essentially loses no copper in compari-
; son to the latter's 1 to 3~ loss; ~he invention processreturns effec~tively all of the metallic zinc content of the
b~ass skimmings in contrast to none xeturned-by the manufac-
turers; the invention pxocess converts the zinc ~xide to a
~0 by-product in c~ntrast to treating the zinc oxide as a waste
pr~du~t, and processing c~t of ~he invention procesq i9 a
fraction o the smelting and reining aharges of the~manu~ac-
turers. Ignorin~ the value Q~ the zinc by-pxoduct derived
from the zinc oxide (wh~ch can typiaally cons~itute ~rom 15
to 25% of the weight in the brass skimmin~s), it ha~ been
estimated that the lnvention pr~ce~s can typically save the
mill or foundry ~150 per ton an~ ~ore (wher~ several mills
generate in excess of 400 tons per mon~h of brass ~kLmmings).
Figure 2 shows a somewhat sim~lified apparatus
~ox practicing the inven~ion. The reaction ves~el 10, feed
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12, agitator 22, etc. are es~entially th~ same; however, the
valve 32 for the settled ~olid~ has been omitted altogether.
The larger solids settle directly into the lower intake
portion of the screw conveyor 40a. They are elevated above
the common liquid level of the vessel 10 and of the conveyor
40a against a countercurrent of the acid introduced remotely
by pipe 18. Any water needed for the reaction can advantag-
eously be added by a pipe 20 (al~o remotely connected to
conveyor 40a) ~o as to unction additionally as a preliminary
xinse upstream of the acid pipe 18. Feed pipe 52 carries
~he ~ettled material from the delivery end of conveyor 40a
t~ the wash tank 42 and the receivlng end of the wash screw-
conveyor ~Ob. As the s~lid~ are again elevated, they are
washed ~f liguid contamlnants and any remaining fine~ in a
countercurrent of water (all of which overflow weir 43).
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